In a spark plug for an internal combustion engine, a center electrode is held generally inside a cylindrical insulator. That is, the center electrode is held inside the insulator so that the tip end portion protrudes. Here, the center electrode has a locking portion that is locked from the base end side to a step portion formed on the inner peripheral surface of the insulator, and an electrode head that is formed on the base end side of the locking portion. Electrically conductive glass is filled inside the insulator so as to be located on the base end side of the center electrode. A resistor and a stem are disposed inside the insulator on the base end side of the conductive glass. In this manner, the center electrode is electrically connected to the stem via the conductive glass and the resistor.
Here, the conductive glass is bonded to the electrode head of the center electrode. In order to increase the bonding strength between the electrode head and the conductive glass, PTL 1 proposes a technique to provide a concave portion on the base end surface of the electrode head.